Three days after the Boston Marathon bombings prompted them to pull down their ads and stop campaigning, the candidates for US Senate in Massachusetts ­began to stump for votes again, rejoin­ing a race that has been ­upended by the deadly attack.

Two of the Republican candidates, state Representative Daniel B. Winslow and former US attorney Michael J. Sullivan, took part in a candidates’ forum Thursday night in Taunton, hours after President Obama led an emotional service for the bombing victims at the Cathedral of the Holy Cross in ­Boston’s South End.

On Friday, Winslow, Sullivan, and the third Republican candidate, former US Navy SEAL ­Gabriel E. Gomez, have a full schedule of campaign activities planned, including meetings with seniors and veterans on Cape Cod and with GOP activists at a Swampscott steakhouse.

The Democratic candidates, US Representatives Stephen F. Lynch and Edward J. Markey, plan to ­resume campaigning Saturday, with schedules that include a ­forum at Roxbury Community College.

All five campaigns said they are determined to strike a serious tone and focus on issues, since they are cognizant that voters will be less tolerant of personal sniping and petty disagreements in the after­math of the attack.

But with less than two weeks remaining until the April 30 primary, the pledge of civility may not hold, particularly in the race for the ­Republican nomination, which has been marked by frequent exchanges of charges and counter­charges.

By Thursday afternoon, some of the GOP campaigns were accusing one another of violating the suspension of campaign activities declared ­after Monday’s bombings.

“We want to err on the side of more, not less, respect for the solemnity of the day,” Alcivar said. “Our opponents have, ­apparently, opted not to do that.”

Sullivan’s campaign manager, Paul Moore, said the brochure was approved last week, before the bombings on Monday, and the campaign was ­unable to contact the vendor in time to stop it from being sent.

Moore, meanwhile, accused the Gomez campaign of failing to pull a negative television ad off the air this week. Alcivar said that if the ad was running, it was only because of a delay on the part of the television station’.

The candidates will all have to negotiate a landscape fraught with political peril as they try to engage voters who are still shaken by the attack.

The campaigns have not said when they plan to resume advertising, but Sullivan’s camp said it would no longer run a television ad that attacked Gomez for asking Governor ­Deval Patrick to appoint him to the interim Senate seat.

“We want to focus on the larger issues that affect everyone, and not get down in the weeds,” Moore said.

In a statement Wednesday, Winslow recalled how his house was bombed by an unknown attacker in 1990, when he was serving in Norfolk town government. Winslow said he and his pregnant wife were not home at the time, but their sense of security was shattered.

“The bombing was the culmination of a long series of harass­ment and vandalism to try to scare and bully me and my fellow elected town leaders,” Winslow wrote. “While I certainly was frightened, I refused to quit town government, and I said at the time that I’m not about to let a bomber interfere with the operation of government.”

In that same spirit, Winslow said, he would resume his campaign after Thursday’s interfaith service at the cathedral.

“As I did in 1990, with firm resolve for the importance of the decisions that lie ahead and with a heavy heart for the innocents killed and hurt by this senseless violence, I have directed my campaign with all sensitivity and respect to ­recommit to our democratic process,” Winslow said.

On Friday, he and Sullivan plan to take part in a debate on WBZ-AM (1030) at 9 p.m.

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